1. Field of the Invention
The present invention is directed to a method of etching anodic foil for use in the manufacture of electrolytic capacitors and more particularly to a method of creating porous anode foil for use in multiple anode stack configuration electrolytic capacitors. The resulting foil reduces the equivalent series resistance (ESR) of multiple anode stack configurations without sacrificing capacitance. The invention further relates to an electrolytic capacitor incorporating the etched anode foil of the present invention for use in an implantable cardioverter defibrillator (ICD).
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors arc used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an aluminum electrolytic capacitor is provided by the anodes, a clear strategy for increasing the energy density in the capacitor is to minimize the volume taken up by paper and cathode and maximize the number of anodes. A multiple anode stack configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. Energy storage density can be increased by using a multiple anode stack configuration element, however, the drawback is that the equivalent series resistance, ESR, of the capacitor increases as the conduction path from cathode to anode becomes increasingly tortuous. To charge and discharge the inner anodes (furthest from the cathode) charge must flow through the outer anodes. With typical anode foil, the path through an anode is quite tortuous and results in a high ESR for a multiple anode stack configuration.
The conduction path from the cathode to the inner anodes may be made less tortuous by providing pores in the outer anode foil. In this manner, charge can flow directly through the outer anodes to the inner anodes. Thus, the use of porous anode foil can combat the increase in ESR resulting from the use of a multiple anode stack configuration. However, the type of pore structure in the anode foil affects the resulting ESR. A macroscopic pore size and spacing, such as can be obtained by mechanical means, as shown in PCT Published Application WO 00/19470, is not as desirable for the reduction of ESR. This type of structure reduces the capacitance of the anode foil and is not optimal in reduction of ESR. Therefore, there is a need in the art for a method of creating porous anode foil for use in multiple anode stack configuration electrolytic capacitors that minimizes ESR while maintaining high capacitance.
The present invention is directed to a method of creating porous anode foil for use in multiple anode stack configuration electrolytic capacitors, producing a pore structure that is microscopic in pore diameter and spacing, allowing for increased energy density with a minimal increase in ESR. The etched anode foil is made porous to the fill electrolyte by subjecting the etched anode foil to an electrochemical drilling solution which selectively etches pores though the foil. The method according to the present invention produces anode foil with improved porosity which may be used in a multiple anode stack configuration electrolytic capacitor allowing for increased current density without an excessive ESR increase.
A three step etch process is used according to the present invention. Initially, an anode metal foil is etched, according to a conventional etch process, to produce an enlargement of surface area, preferably and enlargement of at least 20 times. The etched foil is then placed into the electrochemical drilling solution of the present invention. Alternatively, the etched foil may be masked, so that only small areas of the etched foil are exposed, prior to being placed in the electrochemical drilling solution. A DC power supply is used to electrochemically etch the masked or unmasked foil in the specialized electrochemical drilling solution such that pores on the order of about 1 micron to about 1000 microns in diameter are produced through the foil increasing its porosity, to provide a significant reduction of ESR in the final multiple anode stack configuration capacitor build. The electrochemical drilling solution of the present invention consists of 1 to 15% by weight sodium chloride, preferably 5% by weight, and 10 to 5000 PPM of a surface passivator, such as sodium nitrate or phosphoric acid, preferably 500 PPM. Finally, the foil is widened and formed to the intended use voltage according to conventional widening and forming processes. As used herein, the phrase xe2x80x9cwidening the foilxe2x80x9d is intended to mean widening the pores that were generated in the foil during etching and/or drilling processes. When used in conjunction with an electrochemical etch preceding it and an electrochemical widening step following it, the electrochemical drilling process of the present invention results in an electrically porous foil which maintains the high capacitance gain produced by the etching and widening steps alone, but, when used in a multiple anode stack configuration, exhibits a reduced equivalent series resistance.
The anode foil of the present invention is suitable for use in an electrolytic capacitor with a multiple anode stack or wound roll configuration, after forming a barrier oxide sufficiently thick to support the intended use voltage. The electrochemical drilling step of the present invention produces a pore structure in the anode foil which is microscopic in pore diameter and spacing.
The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.
FIG. 1 is an SEM photograph of the surface of an electrochemically drilled anode foil according to one embodiment of the present invention.
FIG. 2 is an SEM photograph of the surface of an electrochemically drilled anode foil according to another embodiment of the present invention.
FIG. 3A is a mask pattern according to one embodiment of the present invention.
FIG. 3B is a mask pattern according to another embodiment of the present invention.
FIG. 3C is a mask pattern according to another embodiment of the present invention.
FIG. 3D is a mask pattern according to another embodiment of the present invention.